There is no actual measurment for photosynthesis but most authors measure it by the quantity of fructoso-6-phosphate(the first photosynthesis product) produced/time unit. This can afterwards be turned into sucrose or starch.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter